Let’s watch the sky. We will focus on one common and one rare sky phenomenon. The rare one is the stunning CZA, an “upside-down rainbow” whose colors are often astoundingly intense. And even more amazing than its brilliance is its positioning: A CZA can only appear overhead! While actual rainbows are never high up, the CZA is never low down.
The preferred time is when the Sun is about 20 degrees high, which nowadays means around 10 a.m. and then again around 2 p.m. But it often materializes a full hour from those times. You’ll always remember the experience if you spy the CZA once.
So, whenever the sky is mostly clear, but has thin or barely-there high clouds, look straight up. Then slowly move your eyes downward in the direction of the Sun. Bingo: There are the spectral colors forming a “smile” rather than a rainbow’s “frown.” They’ll always pop out better through sunglasses.
The full name is the circumzenithal arc. Around here, it probably appears once a month.
But now consider the most common dramatic apparition of the day or night: It’s the 22-degree halo. Seen encircling the Sun or the Moon, a halo is bigger than people remember or expect. In our area, they typically appear at least once a week.
What’s that? You haven’t seen one in years? That’s because you don’t look up very often. The optimum time is whenever the sky has delicate high-altitude cirrus clouds, or else thin sheets of high cirrostratus. Both are semi-transparent clouds made of ice crystals rather than water droplets. If you simply look up from time to time, you’ll easily see when the clouds are thin and high. Then look around the Sun. Or Moon.
The present cold season offers a profusion of ice-crystal phenomena, but the halo – with red always on the inside of the ring – is commonplace. And the ring is always the same size. Maximally spread open your hand and hold it out at arm’s length, and close one eye. If you place your thumb at the Sun, your pinky-tip will mark the position of the halo: Count on it.
Folklore says that such a ring around the Sun or Moon portends the arrival of a storm within 24 hours. There’s truth to that. A frontal system that may bring rain or snow typically starts by shoving high-altitude clouds over us: the cold ice-crystal clouds. Although it may be a glancing blow with no rain, these cirrus clouds usually give way to lower-altitude clouds, and then even lower and thicker ones that actually deliver the precipitation. The timeframe is indeed 24 hours or less.
Next time you see a halo, look at the right and leftmost edges, at the 3 and 9 o’clock positions, and look for a bright spot at either place. Yes, that’s a sundog, or parhelion. The two require the plural: parhelia.
Next, look at the top of the halo and see if there’s a sort of bowl-shaped phenomenon balancing there. If so, you’ve now observed the upper tangent arc. It’s unique in that it changes its appearance depending on how high up the Sun is.
This can begin the habit of routinely watching the sky, which ultimately will uncover a universe of colorful phenomena. Some are very rare; I’ve only seen the parhelic circle twice in my life, and only once saw a vivid moonbow: a rainbow created by moonlight. The CZA is rare enough to be truly exciting, too. But it all usually starts with the most common of them all: the 22-degree halo.
Want to know more? To read Bob’s previous columns, click here. Check out Bob’s podcast, Astounding Universe, co-hosted by Pulse of the Planet’s Jim Metzner.